Europe would face the "great danger of a real shooting conflict" if Russian forces moved beyond Crimea to enter the main part of eastern Ukraine, William Hague has said as he accused Vladimir Putin of a major miscalculation.

As the foreign secretary warned of another "frozen conflict" in Europe, the energy secretary, Ed Davey, said gas prices could increase if the Ukraine crisis escalated into a military conflict.

But the foreign secretary, who said Putin had implemented carefully prepared plans to assume control of Crimea, acknowledged none of the options on the table – diplomatic pressure and economic sanctions – would be able to remove Russian forces from the Black Sea peninsula.

Asked by the BBC's Europe editor, Gavin Hewitt, what would happen if Russian troops went beyond the Black Sea peninsula to enter "mainland" eastern Ukraine, Hague said: "There would be far reaching trade, economic and financial consequences. It would bring the great danger of a real shooting conflict. There is no doubt about that."

Asked whether Britain and the EU would advise the Ukrainians not to take up arms against the Russians, Hague said: "We have commended all of their restraint so far. It is not really possible to go through different scenarios with the Ukrainians and say: in these circumstances you shoot and in these you don't. We have commended their restraint. They have not risen to any provocation from Russia."

The warning from Hague came shortly after Davey told Britain's energy companies not to seek to make profits from the Ukraine crisis, though he acknowledged gas prices would increase if the crisis escalated.

Ukrainians hold a demonstration in support of a unified Ukraine in the Crimean city of Bakhchisaray. Photograph: Spencer Platt/Getty

The energy secretary told the Andrew Marr Show: "We use Norwegian gas and we have a lot of gas imported on ships – liquefied natural gas. So our security of supply on gas – people shouldn't be worried about that.

"The companies who supply gas and electricity tend to buy their gas forwardly. They buy it 18 months in advance so they shouldn't be using it as an excuse to put up people's prices. They hedged quite rightly. But we have seen that when this crisis broke there was a spike in oil and gas prices. They have now come down.

"But if there was an escalation, if we saw military conflict, if that conflict went on for months and months and months, there could be an impact on prices."

Hague said he believed Putin would eventually be seen to have made a "big miscalculation" as the EU pivots away from Russia, particularly in the energy sphere. But he admitted that none of the proposed EU measures against Russia, to be introduced on a graduated basis if Moscow refuses to change tack, would remove Russian forces from Crimea.

The foreign secretary said: "None of these things force a Russian withdrawal from Crimea. That is well understood. But they will raise the cost to Russia over time."

But the foreign secretary, who rejected next Sunday's planned referendum in Crimea, said there was no "tacit acceptance" of the Russian occupation of Crimea. Some of the sanctions identified in the first phase of the EU's action will be triggered if Moscow refuses to discuss the long-term future of Crimea with Ukraine.

"This is the creation of another frozen conflict in Europe like Abkhazia, that is part of Georgia, like Transnistria, that is part of Moldova. There absolutely isn't a tacit acceptance."

Douglas Alexander, the shadow foreign secretary, said: "The priority in Ukraine must be a de-escalation and a diplomatic resolution to the crisis.

"So I welcome that the foreign secretary made this clear to the BBC this morning when he confirmed that all economic and diplomatic options should remain on the table in seeking to achieve that.

"It is also vital that the UK, along with EU allies, sets out a clear timetable for taking further economic and financial measures if Russia fails to change course in the days ahead.

"The UK should also work with the group of the worlds seven largest economies to agree to suspend Russia from the G8 if it refuses to agree a diplomatic resolution to the crisis."